Experimental Study On Shrinkage And Creep Of Concrete Filled Steel Tube

With the extensive use of CFST(concrete filled steel tube)in high-rise buildings and large span bridges and other buildings,the shrinkage and creep of concrete and the redistribution of internal force after the system change are becoming more and more serious,which directly affect the durability of concrete filled steel tube structures.Therefore,the shrinkage and creep problem of CFST has become a vital factor that need to be considered in engineering.The interaction between the core concrete and the steel tube in CFST is complicated and the core concrete is in three-dimensional stress state,which lead to the redistribution of internal forces in the core concrete of CFST is more complicated than that of ordinary concrete.Therefore,it is necessary to study the shrinkage and creep of concrete filled steel tubes.Based on the test of shrinkage and creep of CFST,the following research work was carried out in this paper.(1)The shrinkage strain rule of CFST is obtained through 6 groups of shrinkage tests of concrete filled steel tube,and the relationship between shrinkage strain of concrete filled steel tube and time is analyzed,then the influence of steel ratio and concrete grade steel on shrinkage strain of concrete is analyzed.It’s very small due to no humidity exchange of CFST.Based on the shrinkage model of ACI 209,GL2000 and MC90,three kinds of models for calculating shrinkage strain of CFST are proposal,which are validated with experiment that the six groups of shrinkage test values.(2)Through the creep test of 18 CFST under the long-term constant load action,the relationship curve of concrete filled steel tube core strain with time under long term load is obtained,and the effect of steel ratio and concrete grade on the initial strain and strain increment of core concrete is analyzed,and the creep increment of CFST is given.The influence of initial strain,steel content and concrete grade on creep increment and creep coefficient are analyzed,and the creep coefficient of ACI 209,GL2000 and MC90(D65)creep model is improved by two different ways.(3)The variety between the calculated creep coefficient of the modified creep model and the three different classic creep test values are compared and analyzed.Analysis results show: the calculated values of the creep coefficient of the first modified ACI 209 creep model and the creep coefficient of the second kind of modified D65 creep model are all in good agreement with the previous 2 groups of classical test values;in the third classical tests,there is a great difference between the calculated values of the first modified ACI 209 creep coefficient and the experimental values,and the calculated values of the second modified D65 creep system are in good agreement with the experimental values.The second kinds of modified D65 creep models can be used for calculating creep coefficient of CFST.